Monoclonal antibodies often achieve their therapeutic benefit through two binding events. First, the variable domain of the antibody binds a specific protein on a target cell, for example, CD20 on the surface of cancer cells. This is followed by recruitment of effector cells such as natural killer (NK) cells that bind to the constant region (Fc) of the antibody and destroy cells to which the antibody is bound. This process, known as antibody-dependent cell cytotoxicity (ADCC), depends on a specific N-glycosylation event at Asn 297 in the Fc domain of the heavy chain of IgG1s, Rothman et al., Mol. Immunol. 26: 1113-1123 (1989). Antibodies that lack this N-glycosylation structure still bind antigen but cannot mediate ADCC, apparently as a result of reduced affinity of the Fc domain of the antibody for the Fc Receptor FcγRIIIa on the surface of NK cells.
The presence of N-glycosylation not only plays a role in the effector function of an antibody, the particular composition of the N-linked oligosaccharide is also important for its end function. The lack of fucose or the presence of bisecting N-acetyl glucosamine has been positively correlated with the potency of the ADCC, Rothman (1989), Umana et al., Nat. Biotech. 17: 176-180 (1999), Shields et al., J. Biol. Chem. 277: 26733-26740 (2002), and Shinkawa et al., J. Biol. Chem. 278: 3466-3473 (2003). There is also evidence that sialylation in the Fc region is positively correlated with the anti-inflammatory properties of intravenous immunoglobulin (IVIG). See, e.g., Kaneko et al., Science, 313: 670-673, 2006; Nimmerjahn and Ravetch., J. Exp. Med., 204: 11-15, 2007.
Given the utility of specific N-glycosylation in the function and potency of antibodies, a method for modifying the composition of N-linked oligosaccharides and modifying the effector function of antibodies would be desirable.
Yeast and other fungal hosts are important production platforms for the generation of recombinant proteins. Yeasts are eukaryotes and, therefore, share common evolutionary processes with higher eukaryotes, including many of the post-translational modifications that occur in the secretory pathway. Recent advances in glycoengineering have resulted in cell lines of the yeast strain Pichia pastoris with genetically modified glycosylation pathways that allow them to carry out a sequence of enzymatic reactions, which mimic the process of glycosylation in humans. See, for example, U.S. Pat. Nos. 7,029,872, 7,326,681 and 7,449,308 that describe methods for producing a recombinant glycoprotein in a lower eukaryote host cell that are substantially identical to their human counterparts. Human-like sialylated bi-antennary complex N-linked glycans like those produced in Pichia pastoris from the aforesaid methods have demonstrated utility for the production of therapeutic glycoproteins. Thus, a method for further modifying or improving the production of antibodies in yeasts such as Pichia pastoris would be desirable.